Field of the Invention
The present invention relates to an epitaxial group-III-nitride buffer layer structure on a heterosubstrate. It also relates to a device structure, in particular to a layer structure of a transistor, a field-effect transistor (FET), a high-electron-mobility transistor (HEMT), in particular a normally-on or normally-off HEMT or a metal-insulator-semiconductor (MIS)HEMT, a Schottky diode, or a P-I-N structure.
Description of the Background Art
Most group-III-nitride based device structures, in particular transistor structures used nowadays for radio frequency (RF) or high-voltage (HV) power-conversion devices, are fabricated on a heterosubstrate, for example, on a substrate of a material other than a group-III-nitride material, such as a Si, SiC, or Al203 (Sapphire) substrate. The ability to use Si as a heterosubstrate is particularly advantageous because it allows using relatively cheap wafers having large industry-standard diameters, and because it forms the basis for monolithically integrating group-III-nitride devices into silicon-based integrated circuits made by a CMOS or related technology.
Such epitaxial group-III-nitride layer structures grown on a heterosubstrate, however, require a sophisticated buffer layer structure between the substrate and the active layer(s) in order to manage stress and defects within the crystalline structure.
In order to control the resistive properties of the buffer layer structure in a group-III-nitride layer structure, Fe doping has been widely used. However, the use of Fe has some disadvantages. More specifically, Fe doping results in an undesired tilt and twist of the crystallite structure, as can be revealed by x-ray diffractometry (XRD). Furthermore, the inventors found that Fe, when provided as a dopant during the growth of the epitaxial layer structure, segregates towards a channel layer, which in operation carries a two-dimensional electron gas, hereinafter also referred to in short as 2 DEG. The presence of Fe in the channel layer is detrimental for achieving a desired high electron concentration in the 2 DEG. Finally, Fe doping causes an undesired Fe contamination of the reactor used for deposition of the group-III-nitride layer structure. This causes an undesired Fe background doping, typically in concentrations up to about 1017 cm−3 in nominally undoped upper HEMT-device layers and on the wafer surface. Since the presence of Fe induces traps for charge carriers, an unintentional Fe doping reduces a dynamical behavior of the on-resistance Ron of group-III-nitride based HEMT devices. Due to the contamination risk, Fe doping is not considered compatible with wafer processing in CMOS processes. This forms an obstacle for an integration of the fabrication of group-III-nitride devices into existing, well-established CMOS process lines on silicon wafers.
The document U.S. Pat. No. 7,884,393 discloses the use of a homosubstrate in the form of a GaN substrate to achieve an extremely low dislocation density in an epitaxial group-III-nitride layer structure grown on the homosubstrate. The low dislocation density achievable by growth on a homosubstrate makes a carbon concentration in the different layers of the layer structure variable to some degree. By growing on a homosubstrate and controlling a carbon concentration, according to U.S. Pat. No. 7,884,393, the quality of a buffer layer and a channel layer of group-III-nitride field-effect transistors and HEMTs is improved. As an application, U.S. Pat. No. 7,884,393 describes a HEMT structure grown on a GaN substrate, which has single high-resistance buffer layer deposited immediately on the GaN substrate, a single GaN channel layer immediately on the buffer layer, and a single barrier layer immediately on the channel layer. The buffer layer is in different embodiments made of GaN or AlGaN and has a carbon concentration of 4×1017 cm−3 or higher. The highest carbon concentration disclosed for the buffer layer in U.S. Pat. No. 7,884,393 is 2×1018 cm−3. The adjacent channel layer does not form a part of the buffer layer, but forms an active layer of the HEMT. It is made of either GaN or InGAN and has a carbon concentration of not more than 4×1018 cm−3. A lower concentration of carbon in the channel layer is described in U.S. Pat. No. 7,884,393 as desirable for obtaining a high purity and thus a high electron mobility.
Group-III-nitride buffer layer structures grown on heterosubstrates, however, and in particular silicon, have a much higher dislocation density compared to those grown on a homosubstrate. This high dislocation density can currently not be avoided. Typical dislocation densities achieved in state-of-the-art buffer layer structures on heterosubstrates are in the range of 5×107-5×109 cm−2.